Our laboratory is specifically concerned with the isolation of eukaryotic protein synthesis initiation factors required for in vitro formation of a functional 80S polypeptide chain initiation complex and in studying the interaction of these protein factors with ribosomes, initiator Met-tRNAf, GTP and mRNA during initiation complex formation. The following areas are and will be pursued in detail. (a) We will characterize in detail the intermediate steps in the elF-5-mediated joining of 60S ribosomal subunits to the 40S initiation complex to form the 80S initiation complex. The following intermediate steps will be characterized in detail. (i) Mode of interaction of eIF-5 with the 40S initiation complex to catalyze :he hydrolysis of 40S ribosome-bound GTP as well as requirement of eIF-5 for 60S subunits joining with be studied. (ii) Mechanism of release of the eIF-2-GDP complex formed in the elF-5-catalyzed hydrolysis of GTP bound to the 40S initiation complex will be investigated. A new protein factor has been isolated from rabbit reticulocyte lysates and mammalian liver extracts which is required for the release of the elF-2-GDp complex from the ribosomal complex. The protein factor will be purified to homogeneity and the detailed biochemical characterization of the factor as well as elucidation of the mechanism of action of this factor will be studied. (b) We will further characterize elF-5, a monomeric protein of Mr about 58,000, particularly with regard to (i) phosphorylation of eIF-5 in vitro and in vivo. The effect of phosphorylation of eIF- 5 on its biological activity will be investigated: (ii) We have prepared and characterized polyclonal anti eIF-5 antibodies. With the help of anti-eIF-5 antibodies as well as synthetic oligonucleotide probes, we will isolate complete cDNA clone of eIF- 5 and determine and characterize its DNA sequence.